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You Say Mitosis and I Say Meiosis.. PowerPoint PPT Presentation

You Say Mitosis and I Say Meiosis. or How Cells Reproduce. A. Introduction. The ability of organisms to reproduce their kind is one characteristic that distinguishes living things from nonliving matter.

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You Say Mitosis and I Say Meiosis..

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Regulation of the cell cycle

These proteins are important in regulating the cell cycle and cause cell proliferation.

Inhibitors of CDKs are used to treat cancer

E. Mitosis is a continuum of changes.

For description, mitosis is usually broken into five subphases:

prophase,

prometaphase,

metaphase,

anaphase, and

telophase.

By late interphase (after the S phase), the chromosomes have been duplicated but are loosely packed.

The centrosomes with centrioles in animal cells have been duplicated

and begin to organize

microtubules into an

aster (star).

In prophase, the chromosomes are tightly coiled and become visible, with sister chromatids joined together.

The nucleoli and

nuclear membrane

begin to disappear.

The mitotic spindle begins to form and appears to push the centrosomes away from each other toward opposite ends (poles) of the cell.

During prometaphase, the nuclear envelope fragments and microtubules from the spindle interact with the chromosomes.

Microtubules from one pole attach to one of two kinetochores, special regions of the centromere,

kinetochores

In metaphase the sister chromatids line up at the metaphase plate, an imaginary plane equidistant between the poles, defining metaphase.

At anaphase, the centromeres divide, separating the sister chromatids.

Each is now pulled toward the pole to which it is attached by spindle fibers.

By the end, the two poles have equivalent collections of chromosomes.

At telophase, the cell continues to elongate as free spindle fibers from each centrosome push off each other.

Two nuclei begin for form, as the nuclear envelope begins to

reappear.

Cytokinesis, division of the cytoplasm, ends.

Animation

Animation #2

F.Cytokinesis divides the cytoplasm: a closer look

Cytokinesis, division of the cytoplasm, typically follows mitosis.

In animals, the first sign of cytokinesis (cleavage) is the appearance of a cleavage furrow in the cell surface near the old metaphase plate.

On the cytoplasmic side of the cleavage furrow a contractile ring of actin microfilaments and the motor protein myosin form.

Contraction of the ring pinches the cell in two.

Cytokinesis in plants, which have cell walls, involves a completely different mechanism.

During telophase, vesicles from the Golgi fuse at the metaphase plate, forming a cell plate.

The plate enlarges until its membranes fuse with the plasma membrane at the perimeter, with the contents of the vesicles forming new wall material in between.

Animation of cell plate formation

G. Mitosis in eukaryotes may have evolved from binary fission in bacteria

Prokaryotes reproduce by binary fission, not mitosis.

Most bacterial genes are located on a single bacterial chromosome which consists of a circular DNA molecule and associated proteins.

While bacteria do not have as many genes or DNA molecules as long as those in eukaryotes, their circular chromosome is still highly folded and coiled in the cell.

In binary fission, chromosome replication begins at one point in the circular chromosome, the origin of replication site.

These copied regions begin to move to opposite ends of the cell.

The mechanism behind the movement of the bacterial chromosome is still an open question.

As the bacterial chromosome is replicating and the copied regions are moving to opposite ends of the cell, the bacterium continues to grow until it reaches twice its original size.

Cell division involves inward growth of the plasma membrane, dividing the parent cell into two daughter cells, each with a complete genome.

It is quite a jump from binary fission to mitosis.

Possible intermediate evolutionary steps are seen in the division of two types of unicellular algae.

In dinoflagellates, replicated chromosomes are attached to the nuclear envelope.

In diatoms, the spindle develops within the nucleus.

H. How is Cancer Related to Mitosis?

Cancer is uncontrolled mitosis!

Cancer cells divide excessively and invade other tissues because they are free of the bodys control mechanisms.

Cancer cells do not stop dividing when growth factors are depleted either because they manufacture their own, have an abnormality in the signaling pathway, or have a problem in the cell cycle control system.

If and when cancer cells stop dividing, they do so at random points, not at the normal checkpoints in the cell cycle.

Cancer cell may divide indefinitely if they have a continual supply of nutrients.

In contrast, nearly all mammalian cells divide 20 to 50 times under culture conditions before they stop, age, and die.

Cancer cells may be immortal.

Cells from a tumor removed from a woman (Henrietta Lacks) in 1951 are still reproducing in culture.

In 1951, a scientist at Johns Hopkins Hospital in Baltimore, Maryland, created the first immortal human cell line with a tissue sample taken from a young black woman with cervical cancer. Those cells, called HeLa cells, quickly became invaluable to medical research.

They were essential to developing the polio vaccine.

They went up in the first space missions to see what would happen to cells in zero gravity.

Many scientific landmarks since then have used her cells, including cloning, gene mapping and in vitro fertilization.

HeLa karyotypes:

The abnormal behavior of cancer cells begins when a single cell in a tissue undergoes a transformation that converts it from a normal cell to a cancer cell.

Normally, the immune system recognizes and destroys transformed cells.

However, cells that evade destruction proliferate to form a tumor, a mass of abnormal cells.

If the abnormal cells remain at the originating site, the lump is called a benign tumor.

Most do not cause serious problems and can be removed by surgery.

An overview with several animations showing how cancer develops. Click here

In a malignant tumor, the cells leave the original site and impairs the functions of one or more organs.

animation

animation

Treatments for metastasizing cancers include high-energy radiation and chemotherapy with toxic drugs.

These treatments target actively dividing cells.

Researchers are beginning to understand how a normal cell is transformed into a cancer cell.

The causes are diverse.

However, cellular transformation always involves the alteration of genes that influence the cell cycle control system.

How can Natural Killer Cells in Our Immune System help to fight cancer?

See this youtube video

Cancer and gene regulation

http://learn.genetics.utah.edu/content/epigenetics/control/

Where does cancer (uncontrolled mitosis) come from?

Faulty signaling pathways

Mutations in human genome (p53 site)

Click here to learn about the p53 site of the genome and cancer

I. A close look at mitosis as a form of reproduction.

In asexual reproduction, a single individual passes along copies of all its genes to its offspring.

Later in prophase I, the joined homologous chromosomes are visible as a tetrad.

At X-shaped regions called chiasmata, sections of nonsister chromatids are exchanged.

CROSSING OVER LEADS TO GENETIC VARIATION!!!

2. At metaphase I homologous pairs of chromosomes, not individual chromosomes are aligned along the metaphase plate.

In humans, you would see 23 tetrads.

3. At anaphase I, it is homologous chromosomes, not sister chromatids, that separate and are carried to opposite poles of the cell.

Sister chromatids remain attached at the centromere until anaphase II.

The processes during the second meiotic division are virtually identical to those of mitosis.

Mitosis produces two identical daughter cells, but meiosis produces 4 very different cells.

Found on a Biology t-shirt

Animation showing mitosis vs. meiosis

R. Sexual life cycles produce genetic variation among offspring

The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises each generation during sexual reproduction.

Three mechanisms contribute to genetic variation:

independent assortment

crossing over

random fertilization

Independent assortment of chromosomes contributes to genetic variability due to the random orientation of tetrads at the metaphase plate.

There is a fifty-fifty chance that a particular daughter cell of meiosis I will get the maternal chromosome of a certain homologous pair and a fifty-fifty chance that it will receive the paternal chromosome.

Each homologous pair of chromosomes is positioned independently of the other pairs at metaphase I.

Therefore, the first meiotic division results in independent assortment of maternal and paternal chromosomes into daughter cells.

The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number of the organism.

If n = 3, there are eight possible combinations.

For humans with n = 23, there are 223 or about 8 million possible combinations of chromosomes.

Click on animation

Independent assortment alone would find each individual chromosome in a gamete that would be exclusively maternal or paternal in origin.

However, crossing over produces recombinant chromosomes which combine genes inherited from each parent.

Crossing over begins very early in prophase I as homologous chromosomes pair up gene by gene.